[37] Brain metabolism normally relies primarily upon blood glucose as an energy source, but during times of low glucose (such as fasting), the brain will primarily use ketone bodies for fuel with a smaller requirement for glucose. The brain can also utilize lactate during exercise.

Although the human brain represents only 2% of the body weight, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization.[40] The need to limit body weight in order, for example, to fly, has led to selection for a reduction of brain size in some species, such as bats.[41] The brain mostly uses glucose for energy, and deprivation of glucose, as can happen in hypoglycemia, can result in loss of consciousness. The energy consumption of the brain does not vary greatly over time, but active regions of the cortex consume somewhat more energy than inactive regions: this fact forms the basis for the functional brain imaging methods PET and fMRI.[42] These are nuclear medicine imaging techniques which produce a three-dimensional image of metabolic activity.